When an optical fibre is subjected to an external force, the fibre will bend and result in attenuation of the light. This is a drawback in signal transmission for example, although bending of the fibre can be utilized in a manner which enables the fibre to be used as a detecting element. One example of such a force detecting element is found described in the European Patent Specification No. 0188512. This specification teaches an optical fibre comprising a core and cladding which is surrounded by a primary protector and a twisted band which is placed between the cladding and the primary protector. The twisted band causes the mentioned bending when the primary protector is subjected to force.
A fibre cable which is used to detect temperature is described and illustrated in the Japanese Patent Application No. 60-195429. In this case, there is wound around the cladding of the optical fibre a wire which is made of a shape memory metal and which contracts when subjected to a given detecting temperature and therewith exerts pressure on the cable cladding, causing the fibre to bend. Due to so-called Rayleigh-scattering, each molecule in the fibre core reflects light, and the intensity of the reflected light is directly proportional to the intensity of the passing light pulse. The intensity of the reflected light pulse decreases as a result of bending of the fibre. The reflected light pulse can be detected and the location of the bend along the fibre can be established with the aid of so-called OTDR, Optical Time Domain Reflectometry. One drawback with this technique is that it is often very difficult to find a shape memory metal which will cause the cable fibre to bend to the desired extent at a desired temperature. The manufacture of the device itself is also relatively complicated.
A further example of an optical fibre cable for temperature detection is described and illustrated in Japanese Patent Application No. 62-6128. In this case, an optical fibre is attached to a straight rod of shape memory metal with the aid of wire ties which are tied around the rod and the optical fibre. When the device is subjected to a selected detection temperature, the rod will fold into a zig-zag shape and therewith bend the optical fibre.